Cataracts are a major cause of blindness in the world and the most prevalent ocular disease. Visual disability from cataracts accounts for more than 8 million physician office visits per year. When the disability from cataracts affects or alters an individual's activities of daily living, surgical lens removal with intraocular lens (IOL) implantation is the preferred method of treating the functional limitations. In the United States, about 2.5 million cataract surgical procedures are performed annually, making it the most common surgery for Americans over the age of 65. About 97 percent of cataract surgery patients receive intraocular lens implants, with the annual costs for cataract surgery and associated care in the United States being upwards of $4 billion.
A cataract is any opacity of a patient's lens, whether it is a localized opacity or a diffuse general loss of transparency. To be clinically significant, however, the cataract must cause a significant reduction in visual acuity or a functional impairment. A cataract occurs as a result of aging or secondary to hereditary factors, trauma, inflammation, metabolic or nutritional disorders, or radiation. Age related cataract conditions are the most common.
In treating a cataract, the surgeon removes the crystalline lens matrix from the lens capsule and replaces it with an intraocular lens (“IOL”) implant. The typical IOL provides a selected focal length that allows the patient to have fairly good distance vision. Since the lens can no longer accommodate, however, the patient typically needs glasses for reading.
More specifically, the imaging properties of the human eye are facilitated by several optical interfaces. A healthy youthful human eye has a total power of approximately 59 diopters, with the anterior surface of the cornea (e.g. the exterior surface, including the tear layer) providing about 48 diopters of power, while the posterior surface provides about −4 diopters. The crystalline lens, which is situated posterior of the pupil in a transparent elastic capsule, also referred to herein as “capsular sac,” supported by the ciliary muscles via zonules, provides about 15 diopters of power, and also performs the critical function of focusing images upon the retina. This focusing ability, referred to as “accommodation,” enables imaging of objects at various distances.
The power of the lens in a youthful eye can be adjusted from 15 diopters to about 29 diopters by adjusting the shape of the lens from a moderately convex shape to a highly convex shape. The mechanism generally accepted to cause this adjustment is that ciliary muscles supporting the capsule (and the lens contained therein) move between a relaxed state (corresponding to the moderately convex shape) and a contracted state (corresponding to the highly convex shape). Because the lens itself is composed of viscous, gelatinous transparent fibers, arranged in an “onion-like” layered structure, forces applied to the capsule by the ciliary muscles via the zonules cause the lens to change shape.
Isolated from the eye, the relaxed capsule and lens take on a more spherical shape. Within the eye, however, the capsule is connected around its circumference by approximately 70 tiny ligament fibers to the ciliary muscles, which in turn are attached to an inner surface of the eyeball. The ciliary muscles that support the lens and capsule therefore are believed to act in a sphincter-muscular mode. Accordingly, when the ciliary muscles are relaxed, the capsule and lens are pulled about the circumference to a larger diameter, thereby flattening the lens, whereas when the ciliary muscles are contracted the lens and capsule relax somewhat and assume a smaller diameter that approaches a more spherical shape.
As noted above, the youthful eye has approximately 14 diopters of accommodation. As a person ages, the lens hardens and becomes less elastic, so that by about age 45-50, accommodation is reduced to about 2 diopters. At a later age the lens may be considered to be non-accommodating, a condition known as “presbyopia”. Because the imaging distance is fixed, presbyopia typically entails the need for bi-focals to facilitate near and far vision.
Apart from age-related loss of accommodation ability, such loss is innate to the placement of IOLs for the treatment of cataracts. IOLs are generally single element lenses made from a suitable polymer material, such as acrylics or silicones. After placement, accommodation is no longer possible, although this ability is typically already lost for persons receiving an IOL. There is significant need to provide for accommodation in IOL products so that IOL recipients will have accommodating ability.
Although previously known workers in the field of accommodating IOLs have made some progress, the relative complexity of the methods and apparatus developed to date have prevented widespread commercialization of such devices. Previously known devices have proved too complex to be practical to construct or have achieved only limited success, due to the inability to provide accommodation of more than 1-2 diopters.
U.S. Pat. No. 5,443,506 to Garabet describes an accommodating fluid-filled lens wherein electrical potentials generated by contraction of the ciliary muscles cause changes in the index of refraction of fluid carried within a central optic portion. U.S. Pat. No. 4,816,031 to Pfoff discloses an IOL with a hard poly methyl methacrylate (PMMA) lens separated by a single chamber from a flexible thin lens layer that uses microfluid pumps to vary a volume of fluid between the PMMA lens portion and the thin layer portion and provide accommodation. U.S. Pat. No. 4,932,966 to Christie et al. discloses an intraocular lens comprising a thin flexible layer sealed along its periphery to a support layer, wherein forces applied to fluid reservoirs in the haptics vary a volume of fluid between the layers to provide accommodation.
Although fluid-actuated mechanisms such as described in the aforementioned patents have been investigated, currently available accommodating lenses include the Crystalens developed by Eyeonics, Inc. (formerly C&C Vision, Inc.) of Aliso Viejo, Calif. According to Eyeonics, redistribution of the ciliary mass upon constriction causes increased vitreous pressure resulting in forward movement of the lens.
Co-pending, commonly assigned U.S. Patent Application Publication No. 2005/0119740 to Esch et al., which is incorporated by reference herein in its entirety, describes an intraocular lens in which forces applied by the lens capsule to a haptic portion of the lens to induce fluid transfer to and from an actuator disposed in contact with a dynamic surface of the lens.
Another disadvantage of previously known devices is that they oftentimes create spherical aberrations. As is well known in the art, lenses composed of elements having spherical surfaces are easy to manufacture but are not ideal for creating a sharp image because light passing through the elements does not focus on a single focal point. In particular, light that passes through a positive optical element close to the optical axis generally converges at a focal point that is further from the lens than a focal point of light passing through the peripheral portion of the lens, thereby creating under corrected spherical aberration. As a result of spherical aberration in an intraocular lens, all of the light passing through the lens does not focus on the retina resulting in an image that may be blurred and may have softened contrast.
Various devices have been used in optical systems to reduce the effect of spherical aberration. For example, an aperture may be used that limits the ability of light to pass through the peripheral portion of the lens. As a result, the light contributing to the aberration is prevented from passing through the lens. Such a device provides an obvious disadvantage that the amount of light allowed to pass through the lens is reduced. Another way to reduce the effect of spherical aberration is to combine two lenses, one convex and one concave. A still further method of reducing the effects of spherical aberration is to use an aspherical lens. However, such combined lenses and lenses having aspherical profiles are significantly more expensive to produce. In addition, combining lenses requires additional space to house the multiple lenses.
While the lens described in the foregoing Esch application is expected to provide significant benefits over previously-known accommodating lens designs, it would be desirable to provide methods and apparatus for further enhancing conversion of lens capsule movements into hydraulic forces, so as to improve modulation of the lens actuator and dynamic surface.
It also would be desirable to provide methods and apparatus to enhance the efficiency with which loads arising due to natural accommodating muscular action are converted to hydraulic forces.
It also would be desirable to provide methods and apparatus that reduce spherical aberration while maximizing the useful surface area of an accommodating lens design.